The catacombs of Santa Priscilla located in Rome, along the Salaria road, are among the oldest Christian cemeteries.
In the ornament of a tomb is the image considered the oldest representation of Virgin Mary with baby Jesus.

As you can see above the figure of Mary there is a star, on its left a prophet - Balaam or Isaia- indicates the star that prophesies the advent of Christ.

The painting dates back to the third century AD.

As you know in the circuses were  large valleys, with seats all around, used in the ancient roman times for chariot races.  The chariots  were  turning around a central line called ‘spina’.

The most famous hippodrome of the past is the Circus Maximus in Rome. Augustus in 10bc brought an obelisk from Egypt to decorate the ‘spina’.

The obelisks were considered, by both Egyptian and Romans, to represent the connection between the sky and the earth.

The hippodrome was a little model of the cosmos:

The obelisk was the sun, the ground represented the earth and the canal running in the middle of the spina was the sea.

The four factions of chariot raiders represented the 4 seasons.

The early Christian community took care of the burials of their members. Because of shortage and expensive price of the land  (nothing changed in the past 2000 years ) they created  underground cemeteries excavating tunnels in the volcanic rock  where the bodies were placed in niches along the corridors and sealed with clay or marble slabs. 
Each tomb had the name of the deceased and was also decorated with simple but meaningful symbols. The most common Christian symbols used in the catacombs are:

THE FISH : in Greek fish is written  ICHTHYS:  each Greek letter is the initial  of the words : Jesus Christ Son of God Savior.

     I- stands for IESOUS - (Jesus)

     CH- stands for CHRISTOS - (Christ)

     TH- THEOU stands for - ( of God)

     Y stands for UIOS - ( Son )

     S- stands for SOTER - (Salvatore )

- The MONOGRAM OF CHRIST: it is composed of two Greek letters of the alphabet , X ( chi) and P (ro), woven together . They are the first two letters of the Greek word Xristos , that means Christ . -

- The ANCHOR: meaning  the hope in the promise of a  future life .

- A DOVE with an olive branch in its beak : a symbol recalling the  salvation of Noah after the flood.  Consequently it symbolizes the idea of the soul in a heavenly peace 

Paintings and sculptures in Christian religious art contains by many symbols codified in the past century to define particular subject or saints. Below you’ll find a list of the most usual ones.

Aureole: encircles the whole body, belongs to God but is used for Virgin Mary too. 

In florentine Renaissance 'tondo' indicates works of art executed for private buildings.

Blaise was a bishop leaving in the IV century; he had miraculously saved a boy from suffocating, and thus became protector from throat diseases.

The painter, who certainly was a florentine artist active about 1450, had been identified with Neri di Bicci.

Pinacoteca Capitoline Museum